1. Field of the Invention
The present invention relates to a power-on reset circuit and, in particular, to a power-on reset circuit capable of changing a logic level of a reset signal at multiple threshold voltages.
2. Description of the Related Art
Most electronic devices have a power-on reset circuit, which is used for outputting a reset signal to logic components of the electronic device. The logic components include, for example, registers, counters and flip-flops, which are sensitive to starting status during boot-up. During periods with power supply inconsistencies, the power-on reset circuit outputs a reset signal to restart the electronic device.
FIG. 1 is a schematic view of a conventional power-on reset circuit 10. The power-on reset circuit 10 includes a pull-up component 12 and a pull-down circuit 14. The pull-up component 12 is a P-MOSFET MP1 and the pull-down component 14 is an N-MOSFET MN1.
FIG. 2 is a schematic view of a waveform chart of a reset signal and a threshold voltage of the conventional power-on reset circuit 10 illustrated in FIG. 1. Referring also to FIG. 1, the conductive capability of the P-MOSFET MP1 is different from the conductive capability of the N-MOSFET MN1, so while the voltage level of the power supply VDD is 0V, the P-MOSFET MP1 is not conducted. Therefore, a voltage, Reset_f, of a capacitor C1 is 0V and a logic level, Reset, at an output node N2 of the power-on reset circuit 10 is 0.
While the voltage level of the power supply VDD reaches a threshold voltage VRES, the conductive capability of the P-MOSFET is stronger than the capability of the N-MOSFET, so the first logic component X1 may make a transition for changing an outputted logic level. Meanwhile, a logic level at the output node N2 of the power-on reset circuit 10 is 1 and the power-on reset circuit 10 may output a reset signal, Reset, having a high logic level to other circuits, not shown, for resetting the circuits in logic level.
When the power supply VDD is closed, voltage of the power supply VDD may begin decreasing. When the voltage level of the power supply VDD is less than the threshold voltage VRES, the conductive capability of the P-MOSFET MP1 is weaker than the conductive capability of the N-MOSFET MN1, so the first logic component X1 may make a transition for changing an outputted logic level. Meanwhile, the logic level at the output node N2 of the power-on reset circuit 10 may be changed from 1 to 0, so the logic level of the reset signal, Reset, may be 0.
To overcome the disadvantage of the prior art in that the conventional circuit can only change the logic level of the reset signal with respect to a single voltage threshold VRES, the present invention provides a power-on reset circuit which is capable of changing logic level of reset signal with respect to multiple threshold voltages.